$ A = \left[\begin{array}{rrr}1 & -1 & 3 \\ 1 & -2 & 4 \\ -1 & 2 & -1\end{array}\right]$ What are the dimensions of matrix $ A$ ?
Explanation: The first dimension is the number of rows in the matrix. How many rows does $ A$ have? The second dimension is the number of columns in the matrix. How many columns does $ A$ have? So, $ A$ is a 3 $\times$ 3 matrix.